1. Field of the Invention
This invention relates to a process for preparing polypropylene-based film for shrink packaging which possesses excellent heat shrinkability including shrinkability at low temperatures, which can be used for shrink packaging within a wide range of shrink packaging temperatures, and in which the film remains tight and free from looseness for a long period of time after shrink packaging.
2. Description of the Prior Art
It is well known in the art that a variety of thermoplastic resins can have imparted thereto the property of heat shrinkability when the resins are formed into films under molten conditions, cooled, and stretched under heating conditions.
The heat shrinkability of thermoplastic resin films has been widely utilized in so-called "shrink packaging" wherein an article is loosely packed in a stretched film, followed by heating to shrink the film into close contact with the article, thus decreasing the package volume and improving the saleability of the article.
Various kinds of films of the above-mentioned type have been put on the market, including those made of polypropylene, polyethylene, polyvinyl chloride and the like. Of these, polypropylene film has been recently widely employed for the above purpose owing to its excellent inherent properties such as transparency, gloss, moisture-proof property, non-toxicity, etc. However, known polypropylene-based films for shrink packaging have several disadvantages, including: they do not undergo significant heat shrinkage until they are heated to a high temperature; they possess a very narrow temperature range suitable for effecting shrink packaging; and their percentage of shrinkage is greatly changed by even a slight change in the shrinking temperature.
Accordingly, it is essential in the shrink-packaging step to exactly control and maintain the hot air temperature in the shrinking tunnel at a high and uniform level. When the hot air temperature is too low, the film becomes creased and rumpled due to insufficient shrinkage thereof. On the other hand, if the temperature is too high, the film will be melted and become broken. In addition, non-uniformity of the hot air temperature in the shrinking tunnel will disadvantageously produce pock-marked shrinkage irregularities owing to differences in the degree of shrinkage, thus impairing the appearance of the packaged article. Further, a conventional polypropylene-based film for shrink packaging also has the important disadvantage that even if the article is closely and tightly contacted by the packaging film immediately after the shrink packaging treatment, the film will naturally elongate and loosen with the passage of time, thus reducing the tightness and attractive appearance of the shrink-packaged article.
In order to overcome the disadvantages of known polypropylene-based films for shrink packaging, Japanese Patent Publication Nos. 43-26114 and 45-23437 describe methods for preparing films with excellent heat shrinkability by biaxially stretching films of ethylene-propylene copolymers containing a major proportion of propylene. Further, Japanese Patent Publication No. 43-26115 describes a method for preparing shrink-packaging film in which a mixture of (a) a major proportion of polypropylene and (b) a minor proportion of an ethylene-propylene copolymer containing a major proportion of ethylene, is used as the film-forming material and the film is biaxially stretched whereby once the film is shrunk, it does not stretch and loosen, such as would otherwise occur with the passage of time after completion of the shrink packaging.
However, these methods still present some problems under the existing conditions in which the time period for passing the packaged article through a shrinking tunnel is made very short in order to speed up the shrink packaging step and, hence, the shrink packaging is effected by instantaneously blowing hot air on the shrinkable film. Accordingly, there is a strong demand for the development of a polypropylene-based film for shrink packaging which has an excellent shrinkability at low temperatures and which does not stretch and become loose after completion of the shrink packaging.
It is, therefore, an object of the present invention to provide a process for preparing a polypropylene-based film for shrink packaging which overcomes the above-mentioned disadvantages of the prior art.
It is another object of the present invention to provide a process for preparing a polypropylene-based film for shrink packaging which has excellent low temperature shrinkability.
It is a further object of the present invention to provide a process for preparing a polypropylene-based film for shrink packaging which has a high degree of heat shrinkability over a wide range of shrink-packaging temperatures.
It is a still further object of the present invention to provide a process for preparing a polypropylene-based film for shrink packaging by which uniform and satisfactory shrink packaging is feasible at high speed without exactly controlling the temperatures of the hot air in the shrinking tunnel.
It is an additional object of the present invention to provide a method for preparing a polypropylene-based film for shrink packaging in which the shrunk film does not elongate or loosen after completion of the shrink packaging.